The present invention relates to fuel cells, and more particularly relates to a method of making closed end ceramic tubes for solid oxide fuel cells and the like.
Fuel cells are among the most efficient of power generation devices. One type of solid oxide fuel cell (SOFC) generator has a projected 70 percent net efficiency when used in an integrated SOFC-combustion turbine power system in which the turbine combustor is replaced by a SOFC.
Several different fuel cell designs are known. For example, one type of solid oxide fuel cell consists of an inner porous doped-lanthanum manganite tube having an open end and a closed end, which serves as the support structure for the individual cell, and is also the cathode or air electrode (AE) of the cell. A thin gas-tight yttria-stabilized zirconia electrolyte covers the air electrode except for a relatively thin strip of an interconnection surface, which is a dense gas-tight layer of doped-lanthanum chromite. This strip serves as the electric contacting area to an adjacent cell or, alternatively, to a power contact. A porous nickel-zirconia cermet layer, which is the anode or fuel electrode, covers the electrolyte, but not the interconnection strip. A typical closed end SOFC air electrode tube has a length of about 1.81 m, a diameter of about 2.2 cm and is used in a seal-less SOFC design.
Exemplary fuel cells are disclosed in U.S. Pat. No. 4,431,715 to Isenberg, U.S. Pat. No. 4,395,468 to Isenberg, U.S. Pat. No. 4,490,444 to Isenberg, U.S. Pat. No. 4,562,124 to Ruka, U.S. Pat. No. 4,631,138 to Ruka, U.S. Pat. No. 4,748,091 to Isenberg, U.S. Pat. No. 4,751,152 to Zymboly, U.S. Pat. No. 4,791,035 to Reichner, U.S. Pat. No. 4,833,045 to Pollack, et al., U.S. Pat. No. 4,874,678 to Reichner, U.S. Pat. No. 4,876,163 to Reichner, U.S. Pat. No. 4,888,254 to Reichner, U.S. Pat. No. 5,103,871 to Misawa et al., U.S. Pat. No. 5,108,850 to Carlson et al., U.S. Pat. No. 5,112,544 to Misawa et al., U.S. Pat. No. 5,258,240 to Di Croce et al., and U.S. Pat. No. 5,273,828 to Draper et al., each of which is incorporated herein by reference.
The primary requirements of the closed end of the air electrode for commercial applications are that it has properties that are similar to those of the air electrode tube wall and can be rapidly fabricated, preferably in a high-volume manufacturing facility.
Different techniques have conventionally been used to form the closed end of the air electrode tube. One method is referred to as the pressed plug technique. This process involves forming a rod of air electrode material by extrusion, inserting the rod into a dried, green tube, and applying a uniaxial load. This technique is problematic in that the load applied to the plug material must be sufficient to achieve an adequate bond between the plug and the tube material, but must not be so great as to break the tube. This method also requires controlled drying in order to minimize the possibility of debonding of the plug from the wall and/or cracking of the plug. Plugs made by this method also require machining of the sintered plugged end. The most common problem found in tubes made with this technique is poor bonding at the plug/wall interface. Furthermore, this technique cannot be used to produce closed end ribbed tubular air electrodes, which are being considered for their potential performance enhancement.
An alternate method that has been used to manufacture air electrode tubes is referred to as the cast plug technique. This method involves inserting a cellulose preform into a dried, green tube in order to define the plug internal radius. An air electrode slurry comprising a water-based suspension of AE particles is deposited or cast onto the preform. Precise control of the plug slurry rheology is required to ensure reproducibility. This assembly is then dried slowly in a controlled humidity and temperature chamber to prevent debonding of the plug from the tube wall or the formation of cracks in the plug. Once the air electrode is dry, it is sintered to the desired density and the plugged end is machined or ground to the proper hemispherical radius and plug thickness. The most common problems found in tubes made with this technique are a large difference in porosity between the tube wall and the plug, and poor bonding at the plug/wall interface. Yield problems associated with this technique do not make it a viable commercial option.
Tubes have also been produced using an extruded closed end technique. This technique utilizes a removable die cap that defines the outer hemispherical radius of the close end. With this die cap in place, material is extruded until the closed end is formed. The extrusion pressure is then reduced to zero and the die cap is removed. Extrusion is started again until the required tube length is obtained. Although this technique is an improvement over past methods with respect to closed end homogeneity, it is a start/stop extrusion process which takes a substantial amount of time to perform. In high volume extrusion manufacturing operations, the homogeneity and reproducibility of the extruded product is enhanced by continuous flow as opposed to repeated application and removal of the extrusion load. Closed ends fabricated using this multi-step extrusion process method are not net shape and require post-sintering machining. Additionally, this technique cannot be used to produce closed end ribbed tubular air electrodes.
The present invention provides a method in which a closed end ceramic SOFC tube is formed by joining a cap to a hollow ceramic tube. The cross-sectional geometry of the ceramic tube may be round, square or any other desired geometric configuration. The ceramic tube may optionally include at least one integral rib. The cap may be flat, hemispherical or any other suitable configuration. The cap and the hollow tube are preferably joined by means of a compound joint, such as a rabbet joint or the like. The closed end tube may comprise an air electrode suitable for use in fuel cells. As used herein, the term xe2x80x9cfuel cellxe2x80x9d includes SOFCs, oxygen/hydrogen generator type solid oxide electrolyte electrochemical cells, solid oxide electrolyte cells, oxygen sensors and the like.
An object of the present invention is to provide an improved method of making a closed end ceramic fuel cell tube.
Another object of the present invention is to provide a method of making a closed end ceramic fuel cell tube. The method includes the steps of providing an unfired ceramic fuel cell tube, bonding an unfired end cap to an end of the unfired ceramic fuel cell tube to form a compound joint, and firing the ceramic fuel cell tube and end cap to form the closed end ceramic fuel cell tube.
These and other objects of the present invention will be more apparent from the following description.